The core of a nuclear reactor comprises a plurality of nuclear fuel bundle assemblies, each assembly consisting of a plurality of nuclear fuel rods. Each fuel rod comprises a circular cylindrical housing, i.e., cladding, which is sealed at both ends by respective end plugs. A plurality of nuclear fuel pellets are stacked in a vertical column inside the cladding to a height less than the length of the cladding, leaving a plenum space above the fuel column. A compression spring is placed inside the plenum for biasing the fuel pellets toward the bottom end plug of the fuel rod. A getter for removing contaminants from the interior atmosphere is conventionally installed inside the plenum.
The cladding serves two primary purposes: first, the cladding prevents contact and chemical reaction between the nuclear fuel and the coolant/moderator; and second, the cladding prevents the radioactive fission products, some of which are gases, from being released from the fuel rod into the coolant/moderator. Failure of the cladding, due to build-up of gas pressure or any other reason, could result in contamination of the coolant/moderator and associated systems by radioactive long-lived products to a degree which would interfere with plant operation.
Zirconium and its alloys, e.g., the Zircaloys, are excellent materials for use as nuclear fuel cladding because they have low neutron absorption cross sections and at temperatures below about 600.degree. F., are extremely stable and nonreactive in the presence of demineralized water or steam, the most common reactor coolant/moderator. Within the confines of a sealed fuel rod, however, the hydrogen gas generated by a slow reaction between the cladding and residual water may reach a level which under certain conditions can cause localized hydriding of the zirconium alloy, accompanied by deterioration of mechanical properties. The cladding is also adversely affected by such gases as oxygen, nitrogen, carbon monoxide and carbon dioxide at all temperatures.
The zirconium alloy cladding of a nuclear fuel rod is exposed to contaminant gases during irradiation in a nuclear reactor even though these gases may not be present in the reactor coolant/moderator and may have been excluded as far as possible from the ambient atmosphere during manufacture of the cladding and the fuel rod. Sintered refractory and ceramic compositions, such as uranium dioxide and others used as nuclear fuel, release measurable quantities of the aforementioned gases upon heating, such as during fuel element manufacture and especially during irradiation. These gases react with zirconium alloy cladding to produce embrittlement, which endangers the integrity of the fuel rod. At high temperatures, water vapor reacts with zirconium alloys to produce hydrogen, which further reacts locally with the zirconium alloy to cause embrittlement. These undesirable results are exaggerated by release of these residual gases inside the sealed plenum, which increases the internal pressure within the cladding and thus introduces unanticipated stresses in the presence of corrosive conditions.
In light of the foregoing, it is desirable to minimize the amount of water, water vapor and other gases in the plenum during reactor operation. One approach is to insert a getter in the plenum. The getter is made of material which reacts chemically with water, water vapor and other gases to remove them from the plenum atmosphere. A getter material in the form of an alloy which rapidly reacts stoichiometrically with water, water vapor and reactive gases is disclosed in U.S. Pat. No. 3,899,392 to Grossman et al.
During loading of the fuel pellets into the cladding, the fuel rod is horizontally disposed. Also, the loaded fuel rods are maintained in a horizontal position when being transported. During transport of horizontal fuel rods, there is a risk that the relatively heavy fuel pellets will move from a compacted position into a non-compacted position.
The fuel rods are installed in the reactor in an upright position. If the fuel column has gaps, these gaps will not close due pellet wedging and friction. The presence of axial gaps would lead to undesirable consequences. Specifically, reactors operate under a pressure on the order of magnitude of 1,000 psi. Initially the pressure on the inside of the cladding is on the order of 200 psi. If a pellet becomes wedged inside the cladding, the misaligned pellet is spatially separated from adjacent pellets. The cladding in the area of pellet separation can neck down in response to the reactor pressure of the reactor, thereby holding the misaligned pellet out of place. Two consequences follow from a fuel rod with misaligned fuel pellets: first, the axial power distribution is altered to give high local powers near the axial gap in the fuel column, thereby causing local overheating of the fuel rod; and second, overstressing or cracking of the cladding can occur.
In view of these difficulties, it is conventional practice to install a compression spring in the plenum, which spring bears against the top of the fuel column to hold the fuel pellets in place during fabrication, and transport. The getter for absorbing residual contaminants in the hermetically sealed fuel rod is inserted inside the compression spring in most fuel rods. Sometimes it is desirable to keep the getter assembly a maximum distance from the top of the fuel column, e.g., by attaching the getter to the compression spring at a predetermined height above the fuel column.